1. Field of the Invention
This application relates to a thermoplastic polymer material based on polyethylene terephthalate (PET), which is in the form of an injection-moulded article. This invention also relates to a method for producing such an article.
2. Description of Related Art
Articles, in particular cooking utensils, are usually comprised of a polyamide-based (PA) material, which has numerous advantages, and in particular that of being capable of being injection-moulded. However, the polyamide (PA) has the disadvantage of yellowing due to thermal oxidation when it is subjected to a temperature over 200° C. (frying conditions), and of becoming stained in the dishwasher, which causes a temperature increase to 70° C., i.e. 30° C. above the glass transition temperature of the PA. Indeed, at 30° C. above its glass transition temperature, a polymer generally has a free volume sufficient for being sensitive to pigment diffusion. It is for this reason that the PA is sensitive to the presence of staining foods in the dishwasher (for example, carrots, tomatoes or blueberries).
As the spatula is a cooking utensil intended to be in direct contact with foods, any yellowing or staining problem is detrimental because it is perceived by the consumer to show unsatisfactory hygiene. This leads the manufacturer of polyamide (PA) cooking utensils to develop only lines having a dark colour, in order to overcome these problems of staining.
It is therefore desirable to replace the polyamide with a less costly material that does not stain or yellow.
As a replacement material polyethylene terephthalate (PET) is known, which is a thermoplastic polymer that is much less expensive than the polyamide, with a thermal resistance that is equivalent to the polyamide (melting point 255° C.). In addition, the glass transition temperature of PET (80° C.) explains its excellent stain resistance in a dishwasher. Moreover, its thermal stability is better than that of the PA, limiting its yellowing under high temperature. The use of light colours is therefore possible.
The production of spatulas necessarily involves injection-moulding, which alone enables forms of variable thickness to be obtained.
For injection-moulding of a thermoplastic material such as PET, PET is typically used in the form of ready-to-inject granules, i.e. compounded granules (available commercially in this form). These granules typically consist of PET resin, on the one hand, and additives necessary for the injection in particular of the mould-release type, and nucleating on the other hand. The compounding of the PET (which is a method of mixing by melting plastic materials and additives) causes the PET to transition to the molten state, which requires complete drying in order to obtain a residual water content of less than 0.02% and prevent hydrolysis of the polymer in the molten state. This hydrolysis is intended to reduce the molecular mass, detrimental to the mechanical properties of the product. A drying cycle of 6 hours at 160° C. is therefore generally required before compounding of the PET: this explains the higher cost of the PET compounds with respect to the PA compounds, even though the PET resin is more economical than the PA resin!
In this field, the French patent FR 2552095 is known, which describes a moulding composition including PET, a nucleation agent (for example a benzoate), a polycaprolactone with a high molecular weight and an unsaturated epoxidised triglyceride, and fillers such as glass fibres, of which the presence implicitly refers to a compounding operation. This reference is also explicit since FR 2552095 teaches that the mixing of components is performed on the extruder, which is the actual definition of compounding.